Drilling of bore holes is conducted for the exploration and production of hydrocarbon fuels, for example in gas and oil exploration and production. The term ‘directional drilling’ is used to describe the process of drilling a bore hole which is directed, for example, towards a target or away from an area where the drilling conditions are difficult. A directional drilling tool generally sits behind a drill bit and forward of measurement tools. The complete system of bit, directional and measurement tools is called the bottom hole assembly or BHA. Currently there are two main types of directional drilling tool, namely positive displacement mud motors and rotary steerable directional drilling tools.
Positive displacement mud motors are placed in the bottom hole assembly behind the drill bit and operate in either a ‘sliding’ or ‘rotating’ mode. When in sliding mode the drill string is held stationary at the surface. Fluid is then pumped through the positive displacement motor which is situated above the drill bit and connected to the drill bit by a drive shaft and universal joint. Generally there is a fixed bend in the collar between the bit and motor in order to offset the drill bits axis of rotation with the axis of rotation of the BHA. The drill bit will then tend to head in the direction of the bend. By controlling the angle of the bend relative to the formation being drilled, the drilling direction can be controlled. However, the angle of the bend can only be controlled from the surface and measurements of the bend position, commonly known as tool face angle, are sent to the surface using some form of up-hole communication device. As drilling progresses the BHA advances forward and the rest of the drill string slides along the well bore, hence the term ‘sliding’.
In order to control the rate of turn of the well bore being drilled, the drill string is rotated from the surface while the motor is rotating the drill bit. This effectively cancels the effect of bend between the motor and drill bit. The drill bit will thus head straight ahead. This is commonly known as rotating.
This method of directional drilling, alternating between rotating and sliding, is slower than continual rotation of the drill string from the surface due to the torque limitation of mud motors, and hence slow rates of penetration are achieved when operating in the sliding mode.
Directional drilling while continually rotating the drill string offers the following advantages: better hole cleaning; smoother well bores, extended reach drilling and higher rates of penetration. However, these tools are often complex in design and hence are costly to manufacture and operate.
For example, UK patent application GB2259316 describes a modulated bias unit for steerable rotary drilling systems. The modulated bias unit comprises one or more pads which press against the side of the formation being drilled to exert a lateral force on the drill bit. By controlling the direction of the force the drill bit can be steered into the required direction. This enables the drill bit to cut across as well as forwards and is commonly known as “push-the-bit”.
Another method involves pointing the bit in the intended drilling direction. For example, International patent application WO0104453 describes a method of deflecting a bit shaft, which runs through the centre of the drilling tool. Deflecting the shaft angles the bit with respect to the remaining parts of the BHA. The bit shaft can be permanently deflected and the position of the deflection controlled, or both the position and magnitude of the deflection can be controlled. These systems typically use a non rotating sleeve which presses against the formation which can be problematic if the hole is drilled slightly over gauge (over size).
“Point-the-bit” drilling can also be performed by contra-rotating a bit shaft in a fixed radius and at a rotation rate equal but opposite to the drill string rotation. For example, International patent application WO9005235 describes such an arrangement. Again this offsets the bit axis of rotation relative to the rest of the BHA and the drill bit will tend to move in the direction of the off-axis offset.